The telecommunications industry presently transmits digital information; that is, data in the form of bits representing either 0s or 1s, over both copper wires and optical fibers, as well as using free space, using many different types of systems to encode and decode the information in many different types of topologies. Although the details of these systems differ, a problem faced by all of the systems is how to accurately receive the transmitted data; that is, the receiver must be able to accurately distinguish 0s and 1s. Reception of the data is complicated because the data may arrive with unknown amplitude and at an unspecified time.
Techniques have been developed to enable the receiver to accurately detect 0s and 1s. For example, the packet may begin with a predetermined sequence of bits which is commonly referred to as the preamble or training sequence. The receiver uses the preamble to align its clock with the transmit clock and to make any necessary adjustments in threshold so that 1s are received with the proper amplitude. Other techniques detect the first pulse by looking for rising or falling edges and then making the necessary clock and threshold adjustments. These techniques are applicable to a wide variety of transmission protocols.
There are communications systems in which data transmission is asymmetric; that is, a system in which one station effectively controls the system. Different terminology is used to represent the stations in such systems. For example, central office or master station may be used with one type of station and local or slave station may be used with the other type of station. In such a system, the master station or central office may send and receive packets of known duration at known intervals; usually there is one master station and a plurality of slave stations. The central office tells the local stations when and for how long they may transmit. Such a system is used in applications such as present and contemplated future fiber-to-the-home systems. Typical data recovery techniques used in these systems are exemplified by those described in the previous paragraph.